Developments in technology have brought the concept of a solar powered aircraft that flies continuously through day/night cycles unassisted for durations of months or years to a point where it now may be considered as something that may be possible in the time frame of the next few years or at least in the next decade.
To achieve the goal of continuous solar powered flight requires that additional improvements be made in a number of related technologies including more efficient and lighter weight solar photovoltaic (PV) arrays, improved means to capture the largest amount of incident solar energy, improvements in the aerodynamic efficiency of the aircraft to reduce electrical power requirements to propel the aircraft, improvements in the aircraft's propulsion system and means for producing thrust, reductions in the weight of the aircraft through the use of improved materials of greater strength while being lighter and while maintaining reasonable structural factors of safety, improved methods of storing the solar energy after direct conversion to electricity by the solar PV cells and higher efficiency methods for removing energy from storage and converting it to useful electrical power.
Presently, technology is being developed along a number of different paths that may converge for use in such a continuous duration solar powered aircraft. For example at the present time, non-concentrating silicon solar arrays which gather solar radiation on a single side are available that operate at up to 24 percent efficiency but to be used for continuous solar powered flight, they still need to be lighter in weight and lower in cost. There are non-concentrating, bifacial silicon solar arrays that accept solar radiation incident on both sides with different electrical conversion efficiencies: presently twenty one percent on the primary surface and fourteen percent on the secondary surface. Again, the bifacial PV cells need to be lighter in weight and higher in electrical conversion efficiency to make continuous solar powered flight feasible.
A solar powered aircraft capable of continuous day/night flight must possess a means to power itself during night periods when solar energy is not available. To accomplish flight during periods of darkness, the aircraft must utilize energy that has been stored during the daytime. Means that are being pursued to store energy include batteries, solar thermal approaches with thermal batteries powering a Stirling engine, solar thermal approaches with a thermal battery powering a Rankine-Brayton engine and closed loop regenerative fuel cells typically powered by hydrogen and oxygen with an electrolyzer producing hydrogen and oxygen from on-board water captured as waste from the fuel cell. For batteries, lithium polymer and lithium-ion batteries appear most promising at this time. With regard to closed loop regenerative fuel cells, an approach that separates an onboard supply of water into hydrogen and oxygen gases during the day using solar energy and stores the gases for later use at night appears to be a most promising approach at this time. At night when electrical power is needed, the hydrogen and oxygen gases are recombined by passing them through a fuel cell to produce electricity with the recombination in the fuel cell producing water for reuse during the next solar day. All of the storage approaches including batteries, solar thermal batteries or closed loop hydrogen-oxygen fuel cells still need substantial development before they would qualify for use in a solar powered aircraft with continuous endurance.
Some other past programs have developed technology that appears attractive for use in a continuous duration solar powered aircraft but substantial improvements need to be incorporated in what is presently available or what has presently been accomplished. But, none of the prior programs has come at all close to what is needed. Present technology is at its infancy in regards to a continuous duration solar powered aircraft.
Recently, one of the more promising programs is the unmanned solar powered Helios aircraft program carried out by AeroVironment Inc. and funded by NASA whose goals were to demonstrate sustained flight at an altitude near 100,000 feet (30.5 km) and to fly non-stop for at least 24 hours, including at least 14 hours above 50,000 feet (15.2 km). The Helios aircraft, in the high altitude version configuration HP01, achieved the first goal in 2001 by reaching an official world-record altitude of 96,683 feet (29.5 km) and sustained flight above 96,000 feet (29.2 km) for more than 40 minutes Unfortunately, the solar powered, flying wing Helios aircraft, in the long flight duration HP03 configuration which was to fly at 50,000 foot (15.2 km) altitude, disintegrated in flight on Jun. 26, 2003 when it suffered a structural failure at an outboard wing location at 2800 feet (0.85 km) altitude just after takeoff; the structural failure attributed to the aircraft flying through wind shear which produced atmospheric turbulence.
It is worthwhile considering the Helios aircraft in more detail for it demonstrated several features that a continuous duration solar powered aircraft might utilize if improved further: bifacial solar cells that accept solar radiation incident on both sides and the use of space age materials to construct a lightweight structure.
The Helios aircraft was an ultra-lightweight flying wing aircraft with a wing span of 247 feet, a gross weight of 1600 pounds and a wing of 8 foot chord and 12 percent airfoil thickness; the upper surface of the wing covered with 62,000 SunPower Inc. bifacial silicon solar cells and the lower surface of the wing covered with a transparent plastic surface to allow solar energy to be incident on the bifacial solar cells from below. The Helios aircraft, configuration HP01, was powered by 14 brushless DC electric motors, 2 HP (1.5 kW) each and configuration HP03 by 10 brushless DC electric motors also 2 HP (1.5 kW) each, distributed along its leading edge and the aircraft was constructed mostly from composite space age materials including carbon fibers, graphite epoxy, Kevlar, and styrofoam; with the styrofoam shaped to form the wing's leading edge and the entire wing wrapped with a thin, transparent plastic skin. Structurally, a large diameter, reinforced carbon fiber hollow tube, located just behind the styrofoam leading edge and running from wing tip to wing tip, carried a majority of the flight loads; along with the wing's ribs and a smaller structural tube member located near the trailing edge of the wing, also running from wing tip to wing tip.
Another recent and promising unmanned solar powered aircraft is the Qinetiq Zephry unmanned air vehicle which is hand launched and has as a goal the carriage of a small communications payload above 40,000 feet (12.2 km) for a two week period. In July 2008, it flew a test flight of 82 hours and 37 minutes duration with a portion of the flight as high as 60,000 feet (18.3 km) while carrying a communication relay payload of 4.4 pounds. The Zephry aircraft has a wing span of about 60 feet, weighs about 66 pounds and has a structure constructed of carbon fibers with amorphous silicon solar cells covering the aircraft's wing. By day it flies on solar power generated by part of the solar array and at night it is powered by rechargeable lithium-sulphur batteries that are recharged during the day using the remaining solar power.
In April 2008, the Defense Research Projects Agency (DARPA) of the U.S. Government instituted Project Vulture by selecting Aurora Flight Sciences, Boeing and Lockheed Martin as contractors for the first phase of a program to develop an unmanned air vehicle able to fly on station at an altitude of 65,000 feet (19.8 km) or higher and perform its mission for five years without interruption. The ultimate goal of the Vulture program is to develop a system capable of carrying a 1,000 pound payload drawing five kW of power with the air vehicle remaining in the required mission airspace 99 percent of the time. During Phase 1, an analytical effort was carried out by the three contractors who conducted trade studies to determine the design concept that best satisfied the operational tasks and optimized the design capabilities. Phase 2 is to be a risk development and testing phase (2009-mid 2012) with Phase 3 to follow in which a full scale aircraft demonstrator capable of staying up for 12 months is to be fabricated.
During the Phase 1 studies, Aurora Flight Science was known to be considering a solar powered unmanned aircraft of unique shape, like a “Z” letter, of modular design where the full size vehicle is assembled in flight by modules that attach to each other; the “Z” shape to be adjustable in flight to maximize solar collection and energy stored in onboard batteries to be used at night. Boeing chose to work with Qinetiq and was expected to use the expertise garnered by Qinetiq in developing technologies for its high-altitude, long endurance solar vehicle, the Zephyr. The plans of Lockheed Martin regarding their proposal were not made public.
Though it is known that Phase 1 was completed by late spring 2009 by all three contractors and that Aurora Flight Science was subsequently dropped from participation in Phase 2, little additional public information has been released regarding the results of the trade studies, about the most promising aircraft configurations or about the most promising energy storage methods.
In the realm of manned solar powered aircraft, the Solar Impulse prototype aircraft, HB-SI-A, was unveiled in Switzerland on Jun. 26, 2009. It weighs 3527 pounds and is intended to demonstrate the ability of a single pilot, manned aircraft to remain aloft for a complete day-night-day cycle. The Solar Impulse prototype is limited to operation at altitudes below 28,000 feet (8.5 km) because it does not have a pressurized cockpit. Its wing span measures about 210 feet and its wing has a chord profile thickness of 17%. It is powered by four 10 HP electric motors with electricity provided by 11,628 solar cells; 10,748 on the wing and 880 on the horizontal stabilizer. During the day, the solar array is used to charge lithium-polymer batteries to enable nighttime operation. Its first test flight is expected in late 2009.
There are two additional non-solar powered aircraft projects that are worth mentioning that have established the present state of the art in fuel cell powered unmanned and manned aircraft. The first is the unmanned, non-regenerative, hydrogen fuel cell powered air vehicle designated the Ion Tiger aircraft, a research program of the U.S. Naval Research Laboratory, which has the goal of staying in the air for 24 hours with a payload of 5 pounds. The Ion Tiger air vehicle is relatively small with a wing span of approximately 20 feet and is powered by a 500 watt fuel cell that relies on technology developed by the automotive industry. The second non-solar aircraft is a manned vehicle employing a 25 kW hydrogen fuel cell for power, the motor assisted glider, Antares DRL-H2, under development by the German Aerospace Center (DRL) and flown for a 10 minute flight in July 2009. The opinion expressed in an article regarding this fuel cell powered aircraft is “don't expect fuel-cell-based jetliners any time soon: actually, the most likely ETA (estimated time of arrival) for such aircraft is never, since fuel cells have a power-to-weight ratio that makes large planes impractical”.
In summary, the technology that is currently available for use in future continuous duration, high altitude, unmanned solar aircraft capable of carrying a payload of 1000 pounds is meager at best. On the one hand, major improvements are required in the materials used to construct such vehicles, in the structural analysis methods used to design them and in developing a better understanding of the loads that they will experience in flight so they don't disintegrate in flight like the Helios vehicle. Major improvements are also needed with regards to the solar arrays that are used on these vehicles; particularly in the need for higher solar conversion efficiencies for the arrays while also reducing the weight per unit area of the array and major efficiency improvements are required in the area of electrical energy storage and for the recovery of energy from storage.
Proposed regenerative electrical power systems utilizing closed loop, hydrogen and oxygen fueled fuel cells with an onboard electrolyzer for replenishment of the hydrogen and oxygen gases using solar energy during the day appear to be paper designs at this time requiring substantial development particularly in the area of energy production per unit weight of the fuel cell. Future development of a continuous duration solar powered unmanned aircraft depends on achieving a variety of incremental improvements in each of a variety of technological areas all related to each other in the design of the aircraft; for instance, a solar array with a higher efficiency means a lighter weight aircraft requiring a reduced wing area and lower electrical power requirements for propulsion.
Any means to increase the effectiveness of the electrical energy production on a continuous duration solar powered aircraft, no matter how small an improvement, is reflected immediately in the aircraft's design either in its size or performance. Increased effectiveness of electrical energy production, especially if that increase in effectiveness is realized with little weight gain, is especially valuable to speed the availability of such an aircraft.
It has now been discovered that the present invention does just that: it provides extra electricity with little associated weight addition by incorporating minor changes in existing features of such an aircraft for the production, collection and storage of the extra electricity. The extra energy is realized from the fact that static charge is continuously formed on parts of the aircraft when the aircraft collides with particles in the earth's atmosphere whether the surface material is metal or a dielectric material. Particles in the earth's atmosphere include, but are not limited to, dust, fog, rain, sleet, snow, ice and volcanic ash particles. In this patent application, the word particle shall include all the preceding particle variants as well as all other types of particles that may be present.
The difference between static charge forming on a metal surface or on a dielectric one is that on a metal surface the charge migrates easily and forms an equipotential surface while on a dielectric surface the charge remains where it is formed because of the low electrical conductivity of the dielectric. The collection of static charge being generated on the dielectric surface is collected in the present invention thru the addition of electrodes to the surface and/or by coating the surface with conducting material that facilitates the migration of the charge to the collecting electrodes. The present invention collects this static charge and uses it to meet immediate aircraft power needs or stores it for future use at night. The extra electrical power produced by the present invention lowers the amount that must be provided by the aircraft's principal source of electrical power generation thus making it easier to create the aircraft. Aspects of the present invention are now described.
A patent search was conducted to identify prior art for cases where static charge is utilized for any useful purpose on an aircraft or in particular, on a continuous duration solar powered aircraft, the subject of the present application. The only prior patent that was found for the useful use of static charge on an aircraft was U.S. Pat. No. 7,592,783 issued on Sep. 22, 2009 to Philip Jarvinen who also is the inventor for the present application. U.S. Pat. No. 7,592,783 is for a P-Static Electrical Power System for an Aircraft that produces electrical energy from collisions of the aircraft with dust, rain, sleet, snow and ice particles. All other prior art that was found was for the elimination of static charge forming on an aircraft because static charge was considered a bad and bothersome phenomena and something to rid the aircraft of as quickly as possible because of its deleterious effects on aircraft communication and navigation systems.
No prior art was found that had any bearing on the present invention for static charge collection and its use on a continuous duration, day/night cycle solar powered aircraft. No prior art was found for the generation and use of static charge formed on an aircraft by the collision of the aircraft with particles in the earth's atmosphere which is the subject of the present application. No prior art was found regarding the use of structural elements of a solar powered aircraft for the storage of static charge which is the subject of the present application. No prior art was found for designing the propeller blades of a continuous duration solar powered aircraft with composite materials to generate static charge thru collision with atmospheric particles as is done in the present invention or for providing electrodes on the surface of the propeller blades to bleed the static charge to the propeller drive shaft and thence to a static charge storage device which is the subject of the present invention.